Download 98 - Radiation From a PCB with Coupling Between a Low

Radiation from a PCB with Coupling
between a Low Frequency and a Digital Signal Traces
Chiharu
Naoto Oka
MITSUBISHI
Information
5-l-l
ELECTRIC
Miyazaki
Corp.
Shuichi
Nitta*
*Faculty of Technology
Tokyo University
of Agriculture
& Technology
Technology R&D Center
Ofuna,Kamakura,Kanagawa,247,Japan
2-24-16 Nakacho,Koganei,Tokyo,184,Japan
Abstract:
Radiation
from a PCB with coupling
between a low frequency signal and a digital signal
not shown in the above paper. In many cases of the
high-density
assembled PCB, traces are routed in
traces is studied
parallel to each other on the different several signal
planes in the PCB with various size of ground
plane.
in this paper.
EM1 increasing
coupling between these traces on a digital
shown experimentally
and theoretically.
traces
are routed
by
PCB is
These
In this paper, coupling
on the same and the different
signal planes in the multilayer
is evaluated
with calculation
shown that evaluation
PCB. Radiated EM1
of crosstalk.
It is
of EM1 level by crosstaIk
is
useful to decide PCB’s structure for EM1 reduction
of a high-density
assembled PCB. It is effective for
EM1 reduction
trace from
plane.
to separate
a high-speed
a low frequency
digital
signal
signal
by ground
IN7230Duc~0N
Downsized
electronic
products
signal
line causes drastic
increase
of EM1 horn
a
digital
PCB. Radiated
EM1 is evaluated
with
calculation
of crosstalk.
Width of PCB’s ground
plane is taken into account in this calculation.
It is
shown that evaluation of EM1 level by crosstalk is
useful to decide PCB’s structure for EM1 reduction
of a high-density
assembled
PCB. From
the
viewpoint of practical application,
it is effective for
EM1 reduction to separate a low eequency signal
require
high-density
assemblies of the printed
circuit boards (PCBs).
These PCBs usually have a ground plane, a power
plane and signal planes. Digital
between traces routed on the
same and the different signal planes is studied. It is
shown by measurement
results
that a coupled
signal traces (clock
and data traces) and low hequency signal traces
(analog signal traces, reference voltage traces and
traces for analog power supply) are placed on signal
planes. These traces are closely routed to each other
on the high-density
assembled PCB. If digital signal
traces couple to low frequency signal traces and
their RF energy couples to I/O circuits, this coupling
results in radiated EM1 [ 11. In the case that these
trace from
a high-speed
digital
signal
by ground
plane.
EPERWENTALMODEL
An example of the PCB studied in this
.shown in Figure 1 (1). Digital
circuits
paper is
and low
frequency circuits are mounted on the PCB (a).
Digital circuits have high-speed
clock and data
signals. Low frequency circuits are analog signal,
analog power supply, reference
voltage, control
signal circuits and so on. A cable assembly connects
traces were routed on the same signal plane in a
PCB with wide ground plane, a study on EM1 level
was reported [2]. Calculation
results of radiation
signal traces &om low frequency circuits to the
PCB(b). Low frequency signal traces and digital
signal traces are closely routed to each other on the
from a cable connected to a PCB were shown but a
measurement result of radiation from the PCB was
high-density
assembled PCB. RF energy
digital signal couples to the low frequency
of the
signal
traces.
A simple model of this PCB is shown in Figure
PCB (2): The victim trace is routed just above
the source trace.
l(2).
This board has two signal traces and a ground plane.
PCB (3): The victim
One trace is a source trace (a model of a digital
signal trace). The other trace is a victim trace (a
the source trace.
PCB (4): The source trace is routed just above
model of a low frequency
signal trace).
the victim
The source
end and is extended
with a resister
trace.
PCB (5): The source trace and the victim
on the same signal plane.
trace is connected to a digital IC at the one end and
terminated
with a resister at the other end. The
victim trace is terminated
trace is routed just above
trace are
at the one
to the outside of the PCB as a
cable section. These traces run in parallel
other and make a coupling section.
to each
155mm
1
1 Oscjllator(25MHz)
4
Cable
B
(sourcei330,“”
(I ) (Viz:e
74ALso4
f
Frequency
circuit
I
I
4
100
4
PCB(a)
PCB(b)
50
100
;r
(1) Top view of the experimental
k
model
(1) An example of the P CB
Dielectric
Material
Victim Trace
Cable Section
Victim
/
L
PCB(l)
Source Trace
Source
Trace
Trace
_
Ground
Plane
PCB(3)
PCB(2)
Coupling Section
-
:Source Trace
n
:Victim
Trace
(2) A simple model of the PCB
Figure
PCB(4)
PCB
and a simple model.
1. An example
of the
PCB(6)
(2) Cross section of coupling
Experimental
models of this PCB are shown in
Figure 2. PdB (1) is used in order to measure
radiation from a PCB without a victim trace and a
cable section. Therefore this PCB has only a digital
signal trace of 1OOmm length. PCBs (2)-(5) are used
in order to measure radiation from a PCB with a
victim trace and a cable section. Lengths of the
coupling section and the cable section are 50mm
and lOOmm, respectively.
PCBs
(2)-(5)
have
different
section.
cross-sectional
structure
section
Figure 2. Experimental model of the PCB.
In each of these PCBs, digital IC is 74ALSO4 logic
IC and oscillator signal is fed to an input of this IC.
The oscillator signal is a periodic trapezoidal wave
of 25MHz fundamental
frequency, that is, the signal
of 25MHz-clock frequency. Width of signal traces is
equal to 0.5mm. The PCB’s dielectric
specific permittivity
E r =4.7.
of the coupling
636
material
has
INFLUENCEOF RADIATIONFROMA CABLE SECTION
EMI INCREASEDBY
A COUPLEDSIGNAL~LINE(V
ICTIM)
A calculated
Radiation
according
from the
to the EMI
the
anechoic
EM1
method
is
standards.
height
based
PCBs (l)-(5)
is measured
Sm-measurement
method in
and
of radiation
from
the
cable
section on the PCB (2) and a measurement result of
radiation ikom this PCB are shown in Figure 4.
chamber.
This
measurement
First
on
EM1
measurement
current on the cable section was calculated
by
equation (2). This equation is derived from equation
the
The PCB is placed on the table of 0.3m
maximum
result
the
measurement
electric
field strength
results
express
in consideration
step of in this
calculation,
radiation
from
(1).
of
height pattern
and directivity.
Electric
field of
horizontal
polarization
is studied in this paper.
Measurement
results of radiation from the PCBs
are shown in Figure 3.
-o-PcB(1)
*PCB(4)
60
, Ex ,_ 2@-’1o-71,1
r
-+PCB(2)
+-PCB(5)
+PCB(3)
Ex : Electric
,
I
(2)
field strength
[v/m].
k : Wave number in free-space [radlm].
r : Radial distance from origin [ml.
I : Signal line length [ml.
I(x) : Current
distribution
located on the x axis [A].
: Frequency [Hz]
f
I; :Current measured
section [A].
0
100
200
300
400 500
Frequency [#Hr]
600
700
on the cable section,
at the center of the cable
800
.
-c- Calculated
Id
3
60 ,
Q
0
--c Measured
I
Figure 3. Measurement results of radiation fkom
the PCBs.
Maximum
level of PCB (1) is,‘;33mV/m
(225MHz).
On the other hand, maximum levels of PCBs.(2)-(5)
are from 49 to 54dBuVlm (35OMHz). From the above
discussion,
it can be understood
that
a coupled
100 200 300 400 500 600 700 800
Frequency
[MHz]
signal trace with a cable section causes drastic
increase of EM1 from a digital PCB. In the case that
a victim trace was routed just above a source trace
(PCB (2)), maximum level of EM1 was observed. The
above phenomena were observed in the case that
coupling and cable sections were short (50mm and
lOOmm, respectively). It is predicted that the longer
The PCB
rectangular
and a cable section were placed in
coordinates as shown in Figure 5. Next,
coupling
the height
pattern
or cable section increases
the PCB furthermore.
radiation
from
Figure 4. Calculated and measured radiati:? @om
-.
..
the PCB (2).
method,
that
in -the EM1 3m measurement
is, the
influence
of the
anechoic
chamber’s ground plane was corrected by using
image theory. Current I, was measured by using
current
at the center of the cable
transformer
section.
terminal voltage of the source trace (output of the
IC) and V4 is a voltage on the victim trace at the
end of the coupling
section
(opposite
side of the
termination).
In this paper, crosstalk
V4N1, that is, far-end crosstalk.
is defined
by
Coupling Section
Mcrostrip Line
v2
Vl
v4
zc
Figure 6. Crosstalk calculation model of the PCB.
Figure 5. Position of the PCB in rectangular
coordinates.
Current
distribution
contributes
on
to the increase
the
Zc is an impedance
trace, that is, input
section
cable
of termination
of EM1 from the PCB.
impedance
of the victim
of the cable section.
This Zc is calculated by the moment method with
wire grid model. ‘A model for calculation and result
of calculated
Zc are shown in Figure
7.
Coupling between the source and the victim traces
causes this current distribution.
Therefore crosstalk
from
the
source
trace
to
the
victim
trace
evaluated in the next section.
The measurement
result
is higher
calculated result because of contribution
distribution
on the ground
plane.
is
than this
of current
Magnetic
/+Tzr+
1o
Segmen
Voltage Skrce
field
measured on the ground plane of PCB (2) was 20dB
(1)Model for calculation
higher than that of PCB (1) at 350MHz. This result
implies that large ground plane increases radiation.
(Study of this is the future issue.)
F200
EMI EVATJJATION BY CROSSTALK
.z 160
Radiation Tom the PCB model is able to evaluate by
applying
numerical
analysis.
In the case of
microstrip
structure
PCB with
thin
dielectric
substrate, many short segments are required and
therefore
long time is necessary for computer
calculation. In practice,
thickness
of dielectric
radiation
crosstalk
from the PCB is evaluated by using
from the source trace to the victim trace.
The circuit
calculation
some of PCBs are of O.lmm
substrate.
In this study,
model
of the
is shown
PCB for’ this
in Figure
6. Vl
crosstalk
is an input
100
50
-4000
0
400
600
800
1000
Frequency[MHz]
(2) Calculated Zc (R+jx).
Figure 7. A model for calculation of Zc (input
impedance of the cable section)
and calculated result.
Capacitance
and
coupling
section
dimensional
inductance
matrices
are calculated
by
boundary
element
in
the
method.
the
two
This
Calculation
are shown
substituting
results of crosstalk by easier method
in Figure
9. In these results,
by
a resistance (330 Q) for impedance Zc,
method can treat various width of the ground plane
and various cross-sectional
structure
of the PCB.
calculation
can be done simply.
Figure 9 are similar to the results
The voltage V4 is calculated
EM1 reduction
by a circuit
simulation
too1 [3].
effect is simply
The results in
in Figure 8. So,
evaluated
by this
method.
Absolute
level of radiation
be predicted
method
with
without
from the PCB model can
the voltage
microstrip
model shown in Figure
EMI REDUCTION
V4 and the moment
structure
of the PCB
7. As the above-mentioned,
Configuration
of layer
in
the
PCB
in
order
to
fist of all, radiation from the PCB is evaluated by
using crosstalk.
Calculation results of crosstalk on the experimental
evaluate effect of EM1 reduction is shown in Figure
10. In the PCB (6), the source and the victim traces
are separated by ground plane. In the PCB (T),
model are shown in Figure 8. From the results in
Figure 2 (PCB (2-5)) and Figure 8, it is recognized
these
that
EM1
reduction
effect
can be evaluated
by
crosstalk.
t
traces
are
separated
PCB(3) *
plane
(100*95mm: power plane or ground plane for analog
circuit is available in practice). In the PCB (B), the
distance between these traces
PCB(2) +
by conductive
PCB(4) -e- PCB(5)
Conductive
is Smm.
Plane
I
:Source Trace
o
:Victim Trace
0
';;i -5
g
-10
j
-15
g -20
CJ -25
-30
o
100 200 300 400 500 600 700 800
Frequency
[MHz]
Figure 8. Calculation results of crosstalk on
the experimental model (1).
PCB(6)
Figure 10. Cross section of the PCBs for evaluation
of EM1 reduction.
Calculation
+PCB(3)
4pCB(2)
*PCB(4)
Measurement
;
-5
g
-10
of crosstalk
results
with
the method
in Figure 11. These
for EM1 reduction.
of radiation
horn these PCBs
are shown in Figure 12. These PCBs shown in
Figure 10 also have good performance
in these
results. Radiation from these PCBs with the cable
section is almost equal to that from the PCB
without the coupling and the cable sections. From
3 -15
j -20
-25
-30 '
0
results
used in Figure 9 are shown
.PCBs have good performance
-PcB(5)
PCB(8)
PW7)
100
200
300
400
500
600
700
Frequenoy[MHzl
Figure 9. Calculation results of crosstalk on
the experimental model (2).
800
these results, it is recognized that EM1 reduction
effect is simply evaluated by crosstalk. For the PCB
(S), it is difticult to obtain enough space in the high-
density assembly because
Configurations
of small PCB’s area.
of PCB (6) and (7) are suitable for
the high-density assembled PCB. EMI increase that
the PCB (7) shows at 750 MHz is caused by
experimentally
and theoretically.
A coupled signal
trace with a cable section causes drastic increase of
resonance with conductive and ground planes.
EM1 from a digital PCB. Maximum level of the PCB
with the coupling and the cable sections is 20dB
higher than the PCB without these sections. In case
that a victim trace is routed just above a source
+PCB(P)
-a~-PCB(~)
*PCB(7)
+pCB(8)
trace, maximum
level of EMI is generated. This
structure must to be avoided on the PCB. From the
viewpoint of practical application, it is effective for
EM1 reduction
trace from
to separate
a high-speed
plane. Radiation
0
200
100
300
400
500
600
700
800
Frequency[dB]
Figure 11. Calculation results of crosstalk of
the PCBs.
a low &equency
digital
from this type of the PCB with the
shown that evaluation
of a high-density
2
-m-PCB(0)
*PCB(7)
by ground
cable section is almost equal that from the PCB
without the coupling and the cable sections.
EM1 increasing by coupling between signal traces is
evaluated by calculation of crosstalk. Moreover, it is
of EM1 level by crosstalk
useful to decide PCB’s structure
tpcB(z)
signal
signal
assembled
is
for EM1 reduction
PCB.
-P'S(8)
I
60 I
Future
issue: Influence
plane and a thinner
EMI.
of various
dielectric
sizes of a ground
substrate
of a PCB on
REFERENCES
[l]
0
100
200
300
400
500
600
700
800
Frequency [MHii
Figure 12. Measurement results of radiation from
the PCBs.
M.I.Montrose,“Printed
Techniques
1996
The conclusion
Compliance”,
Board
Design
IEEE
PRESS,
[2] W.Cui,H.Shi,X.Luo,F.Sha,J.L.Drewniak,T.P.Van
Doren and TAnderson
“Lumped-element
Sections
for Modeling
CONCLUSION
for EMC
Circuit
Coupling
and I/O Lines” IEEE
~~260-265
Between
High-Speed
Digital
1997 Int. Symp. on EMC, 1997,
is as follows:
EM1 increasing
by coupling between
a low
hequency and a digital signal traces was shown
[3] “PCB
Laboratory
Greenfield
Inc., 1991
!
User’s
manual”,
Quantic